A method for retrofitting a nuclear power plant initially including at least one light water reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR), with at least one small modular reactor (SMR)

Abstract

The present invention essentially comprises the steps of disassembling and removing all components of the primary circuit except for the LWR reactor vessel, which is essentially emptied and neutralized, and subsequently replacing some of these components with sub-assemblies each composed of an integrated SMR reactor and a concrete / metal hybrid structure, where this concrete / metal hybrid structure is also used as a reactor pit for the SMR reactor, which is preferably filled with water, and fixing the SMR inside the reactor building and contributing to a third containment barrier, preferably while minimizing damage to the infrastructure of the reactor building. The present invention is related to a method for retrofitting a nuclear power plant.

Description

Technical Field

[0001] The present invention relates to the field of nuclear power plants, and more particularly to the installation bases of nuclear power plants including light water reactors (LWRs), especially pressurized water reactors and boiling water reactors.

[0002] Accordingly, an object of the present invention is to reduce the significant drawbacks in cost and capacity for modifying the installation bases of LWR nuclear power plants.

[0003] The present invention will be described with reference to a nuclear power plant including at least one pressurized water reactor, but the present invention is applicable to any boiling water reactor nuclear power plant or, more generally, any light water reactor (LWR) nuclear power plant.

Background Art

[0004] In a situation where the demand for electricity (competitive, highly available, non-intermittent electricity) is increasing due to energy conversion by decarbonization of use, most of the current installation bases of pressurized water reactor (PWR) nuclear power plants are approaching the end of their operating life for which the reactors were designed and approved.

[0005] FIG. 1 is excerpted from Non-Patent Document 1 and shows the time series of the start of operation of nuclear reactors on a global scale. Most of the 440 reactors that make up the installation base worldwide started operation from 1970 to 1990.

[0006] Depending on the country, extension of the approval rules, or operation rules, an operating life of 40 to 60 years is planned, and all of these reactors will be closed between 2030 and 2050 at the latest. Countries that are using the nuclear power installation base for electricity energy supply and want to maintain that installation base will therefore have to face many investments.

[0007] PWR reactors account for more than 60% of the 440 reactors in the global nuclear power installation base.

[0008] A pressurized water reactor (PWR) includes three cycles (fluid circuits), and the general principle of its normal operation is as follows.

[0009] The high-pressure water in the primary circuit absorbs the energy supplied in the form of heat by the nuclear fission of uranium atomic nuclei and, where appropriate, plutonium atomic nuclei within the reactor core.

[0010] This water, at high pressure and high temperature, typically 155 bar and 300 °C, then enters a steam generator (SG), transfers its energy to the secondary circuit, which also uses water under pressure as the heat transfer fluid. This water in the form of steam, at high pressure, typically about 70 bar, then expands through an expansion member, converting the change in the enthalpy of this fluid into mechanical work and then into electricity by a generator.

[0011] The water in the secondary circuit then condenses through a condenser using the third cycle, the cooling cycle, as a cold source.

[0012] The design principles of the PWR reactor within these three cycles have been substantially the same since the start of operation of the first ones used.

[0013] The main elements of the PWR primary circuit, namely, The reactor building 1 with various functions, including in particular its contribution to the confinement safety function, The reactor vessel 20 installed in the center of building 1, which houses the reactor core C of the reactor, The primary pressurized water circuit 2 including the reactor vessel 20, are shown in FIGS. 2A to 2C.

[0014] These main elements are therefore shared, and their configuration and the number of components vary depending on the output of the reactor.

[0015] The casing of the reactor building 1 can usually consist of multiple thicknesses.

[0016] Therefore, depending on the configuration, the reactor building 1 In a 900 MWe reactor, there is a prestressed concrete wall 10 (Fig. 2A) that functions as an interface with the outside and whose interior is covered by a metal casing 11 with a confinement sealing function. In a 1300 / 1450 MWe reactor, there is a reinforced concrete outer wall 12 and a prestressed concrete inner wall 10 (Fig. 2B) separated from the outer wall 12 by an annular space 13 without material. In a 1650 MWe reactor, there is a reinforced concrete outer wall 12, a prestressed concrete inner wall 10 separated from the outer wall 12 by an annular space 13 without material, and a metal casing 11 on the interior of the prestressed concrete wall 10 (Fig. 2C). It can consist of the above.

[0017] As shown in Fig. 3 cited from Non-Patent Document 2, the primary circuit 2 has the following main components, namely, a reactor vessel 20, a primary loop 21 each containing a primary pump 22 and a steam generator 23, a single pressurizer 24, and consists of the above.

[0018] In Fig. 3, a control rod mechanism and control rods 25 of the reactor core can also be seen.

[0019] Depending on the reactor output, the number of loops can be three for a 900 MWe reactor (Fig. 3) or four for a reactor of 1300 MWe or more.

[0020] The reactor building 1 is therefore sized to accommodate, among other things, all the components of the primary circuit 2.

[0021] Fig. 4 shows the energy transfer cycle (heat and then electricity) of a PWR reactor. In particular, in this Fig. 4, the distribution of the positions of the components with respect to the reactor building 1 having a third confinement barrier function can be seen.

[0022] The fluid connection between the inside and the outside of the nuclear building 1 is provided by lines 30, 31 of the external circuit of the steam generator 23 which leads to the secondary circuit 3 including the turbine 32, the condenser 34, the feed water pump 35, and a heater not shown, which are connected to the generator 33.

[0023] More precisely, for a given steam generator 23, the nuclear building 1 passes through it the so-called hot line 30 which discharges steam from the steam generator 23 for output discharge and supplies the steam to the turbine 32, and the so-called cold line 31 which supplies liquid water to the steam generator 23.

[0024] All existing PWR reactor technologies currently are based on the principle of power plants where their operating life depends on the operating life of the non-replaceable components with the shortest operating life.

[0025] These are mainly components of the primary circuit, and in particular the reactor vessel, which are components that determine the operating life of the power plant due to the activation of materials and the influence of the aging of some of them.

[0026] Therefore, in response to national and especially safety reassessments, attempting to determine the operating life of the entire power plant, initially leading to the predicted operation of existing power plants over a period of 40 to 60 years is mainly due to the age of the reactor vessel.

[0027] Other structural elements of the reactor also age. Among these, it is possible to distinguish two classes: elements that are replaceable during the operating life and elements that are not replaceable.

[0028] Replaceable elements include the steam generator, the primary pump, and the pressurizer.

[0029] In addition to the above primary circuit, non-replaceable elements particularly include civil engineering structures, and the aging of these must be analyzed according to the safety requirements assigned to them. In a PWR reactor with a primary circuit line operating with pressurized water and located overhead, a specific size of the reactor building is required that must provide a function for the safe confinement of nuclear substances in the event of an accident in the primary circuit. In particular, mention can be made of the loss-of-primary-coolant accident studied in a report on the safety of a pressurized water reactor (PWR), which is a virtual accident caused by damage in the casing of the primary circuit. Therefore, there is a direct relationship between the service life of the concrete structure of the building and the safety functions assigned to it.

[0030] The currently emerging technology is the small modular reactor (SMR). These SMR reactors have fundamental advantages over existing PWRs that mainly enable, for safety reasons, simplification of the system and improvement of modularity by manufacturing many components at an off-site plant and transporting them to the construction site.

[0031] SMRs are also flexible due to their low power levels and their ability to insert in that area.

[0032] Therefore, it seems to be a competitive future solution. It has been confirmed that at the time of writing, approximately 70 SMR projects are more or less in progress around the world, and one-quarter of them use mature third-generation (Gen-III) technologies such as the French installation base.

[0033] Some of the SMRs under development propose configurations based on integrating the steam generator or even all components of the primary circuit, particularly the pressurizer and the primary pumps, inside the reactor vessel. These SMRs are called integral SMRs. Apart from the improvement in compactness, integral SMRs have the advantage that they no longer require an overhead pressurized water fluid line, which considerably reduces the risk of accidents related to the rupture of the lines of the primary circuit and the associated consequences.

[0034] For example, the NUWARD (trademark) nuclear power plant project consists of two integrated SMRs with an output rating of 170 MWe each, and all components of the primary circuit 1 are inside the reactor vessel.

[0035] Other integrated SMR projects are under development or research, among which the SCOR project in the name of the applicant of this application with an output rating of 150 MWe to 200 MWe, and the ACP100 project with an output rating equal to 100 MWe can be mentioned.

[0036] Due to the progress of miniaturization of integrated SMRs, their operation has become more complex compared with standard PWRs.

[0037] In fact, the main operations for the operability and structural maintainability of the architecture for the reactor primary circuit are as follows: The operation of fuel loading / unloading that requires access to the inside of the reactor vessel under appropriate radiation protection conditions, The operation for maintenance of equipment that requires access to the equipment, That's it.

[0038] Referring to Figure 3, the loops of the primary circuit 2 of a standard PWR are designed to enable maintenance of each component with little or only a very limited impact on other components. It can be seen that the fuel handling operation is carried out by opening the lid of the reactor vessel 20 without affecting the primary loop 21.

[0039] On the other hand, due to the integration of components, accessing the fuel zone for loading / unloading operations in an integrated SMR may require removal of functional parts of the primary circuit, which is a more difficult task than handling the lid of the reactor vessel.

[0040] In an integrated SMR design, the accessibility to some components varies depending on the configuration and for the arrangement of these components and their functional assembly. For example, in some SMR reactor designs, the operation of loading fuel may require removing some components of the primary circuit.

[0041] Similarly, in some SMR reactor designs, the arrangement of the inlet / outlet tapping of the steam and feedwater make-up lines may vary between a fixed lower compartment and an upper compartment that is removable in the SMR reactor. When the tapping is arranged in the removable upper compartment, it is required to cut the steam and water make-up lines at the inlet of the steam generator before the operation of handling fuel.

[0042] These differences in configuration according to the design are mainly related to the selection of the technology of the internal components, especially the type of heat exchanger, pressurizer, pump... the arrangement and reorganization principle of the architecture inside the reactor vessel (the position and type of the steam generator), especially the reorganization of the so-called critical path. For example, in the SCOR project, the steam generator is on the vertical critical path.

[0043] In summary, the main structural design criteria of the integrated SMR reactor with the view of architectural integration in the reactor building are the requirements for vertical and / or axial accessibility for fuel handling and component maintenance, the methodology for removing / replacing the upper functional components arranged in the removable compartment of the SMR to access the fuel, the arrangement of the tapping of the steam and / or feedwater fluid connections in the removable compartment. That is.

[0044] A pressurized water reactor (PWR) nuclear power plant must be decommissioned when its planned operating life ends.

[0045] In France, at the time of writing this, PWR nuclear power plants have not yet been decommissioned.

[0046] Worldwide, the number of PWR nuclear power plants to be decommissioned is extremely limited.

[0047] Nevertheless, in France, the first power plant, i.e., the Fessenheim power plant, was closed in 2021, and the decommissioning of the Fessenheim power plant has started. EDF, the operator of this nuclear power plant, is formulating a decommissioning plan (Non-Patent Document 2). In particular, refer to the pages of this plan regarding the time series of various stages assumed before, during, and after decommissioning.

[0048] Before the decommissioning itself, the process must be stopped and work must be carried out to optimize the power plant. These preparations for the decommissioning work are To reduce the risks and drawbacks of the facilities, i.e., the discharge of used and new fuels, waste, and waste liquids, the drainage of circuits, the decontamination of some circuits, 99.9% of the radioactivity has been discharged at the time of writing this, To prepare the power plant for the decommissioning work, i.e., the arrangement of access and routes, the adaptation of support functions, especially ventilation, power distribution, and handling, the discharge of specific equipment to create space, To refine the knowledge of the state of the facilities, i.e., the list of hazardous substances, the identification of asbestos, and the sampling for radiation analysis, for the purpose of.

[0049] The intended final state at the completion of the decommissioning is that all buildings are demolished to a depth of 1 meter below the ground surface to become a non-nuclear site.

[0050] In FIGS. 5A to 5D, four consecutive steps of the decommissioning process assumed in Plan [3] (Non-Patent Document 3) and depicted on 5 pages of that plan are reproduced.

[0051] Step 1: This is the electromechanical disassembly, in particular removing and cutting all equipment / components existing in the reactor building 1, especially those of the primary loop 21 (reactor vessel 20, pump 22, steam generator 23...), and, if possible, packaging them as waste for monetization (Fig. 5A). Only the equipment necessary for performing the decontamination work in Step 2 is left intact.

[0052] Step 2: The decontamination of the structures of the nuclear building consists in removing the radioactive contaminants deposited on the inner side of the building, especially the inner wall of the reactor building 1 and the infrastructure 4 therein (Fig. 5B).

[0053] Step 3: Demolition of the buildings including the reactor building 1 and the machine room 5. In a conventional building, demolition is carried out as soon as it becomes useless in any way and can be disassembled. In a nuclear building, demolition cannot be started until the structures are decontaminated in Step 2. The cavity below the ground surface is filled with backfill material consisting of rubble generated by the demolition (Fig. 5C).

[0054] Step 4: Restoration of the site. This is to ensure the compatibility between the state of the ground and future use. Any zone where the buried part 40 of the initial infrastructure 4 is chemically or radiologically marked is subject to the soil management plan (Fig. 5D).

[0055] In a situation where the demand for electricity increases due to the electrification of a large number of energy uses resulting from the decarbonization of energy, most of the nuclear power installation bases will reach the end of their operation within the next 20 years.

[0056] Nuclear power plants have thus already been closed or are scheduled to be closed, although most of the initial investments have not been recovered during their operating life, and nuclear power operators face many investments for the renewal of part or all of the nuclear power plant installation bases.

[0057] The authors of Non-Patent Document 5 are examining the validity of a power plant design based on Pb-Bi FNR type reactor technology in the form of a reactor module.

[0058] The authors of Non-Patent Document 6 mention the theoretical possibility of inserting a Pb-Bi SVBR 75 / 100 FNR reactor unit in a nuclear power plant including a light water reactor at the end of its life.

[0059] Non-Patent Document 7 mentions the renovation of old NPP type power plants with Pb-Bi SVBR 75 / 100 FNR integral blocks. The authors briefly and exclusively mention the economic aspects to be considered for carrying out such renovations, including the spatial insertion of the reactor block in the original reactor building.

[0060] Therefore, there is a need to find a solution that can reduce the investment associated with closing down light water reactors (LWRs), especially pressurized water reactors (PWRs) or boiling water reactors (BWRs). There is a particular need to find a solution that can dismantle them as currently planned.

Prior Art Documents

Non-Patent Documents

[0061]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

[0062] The object of the present invention is thus to address this need at least in part.

MEANS FOR SOLVING THE PROBLEMS

[0063] For this purpose, one aspect of the present invention relates to a method of retrofitting, i.e., modifying, a nuclear power plant initially comprising at least one light water reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR).

[0064] One configuration of a pressurized water reactor (PWR) includes a reactor building housing a reactor vessel, a primary circuit, and a reactor pool, a fuel building, a nuclear fuel handling system for supplying nuclear fuel assemblies from the fuel building to the inside of the reactor vessel and vice versa, a machinery room, a control room, and a nuclear auxiliary building. The method for each reactor comprises the following steps, namely, a / A step of stopping the reactor, including discharging all the fuel assemblies present in the reactor vessel outside the reactor building and completely draining the primary circuit; b / A step of partially electro-mechanically disassembling the reactor, including removing and discharging the components of the primary circuit outside the reactor building, with the exception of leaving the reactor vessel of the reactor as it is in the reactor building, and neutralizing the reactor vessel following the removal of all materials from the inside of the reactor vessel; c / A step of installing at least one removable closed hybrid structure consisting of a metal double skin and concrete injected into the space between two metal walls constituting the double skin, in place of some of the components of the primary circuit discharged in step a / ; Step d: arranging and holding at least one nuclear reactor, called an integrated small modular reactor (SMR), inside each of the hybrid structures installed in step c, the integrated SMR being arranged at a position accessible by a fuel handling system; comprising.

[0065] In the context of the present invention, the expression "nuclear island" has its ordinary meaning, i.e., a nuclear boiler, a combination of fuel-related facilities, and the equipment necessary for the operation and safety of that combination.

[0066] The expression "reactor building" has its ordinary meaning, i.e., a building housing the reactor vessel, all components of the primary pressurized circuit, and some of the circuits for the operation and safety of the reactor.

[0067] The expression "fuel building" has its ordinary meaning, i.e., a building in which handling facilities are installed, particularly storage facilities (fuel assembly storage pools), as well as handling facilities for fresh fuel (awaiting loading into the reactor) and irradiated fuel (awaiting transfer to a processing plant).

[0068] The expression "nuclear auxiliary building" has its ordinary meaning, i.e., a building that protects the auxiliary circuits necessary for the normal operation of the reactor.

[0069] The expression "conventional island" has its ordinary meaning, i.e., a combination of all equipment that enables the conversion of heat generated by nuclear fission in an electrical circuit and then the cooling of these circuits.

[0070] The expression "machine room" has its ordinary meaning, i.e., a building housing a turbine generator unit that serves to convert the steam generated in the nuclear island into electricity, and its auxiliary devices.

[0071] "Neutralization of the reactor vessel" means permanently rendering the reactor inoperable by closing the vessel in a sealed radiation-protective manner, leaving it in its original reactor vessel state, with no fuel material inside, and filling it with an inert fluid for storage.

[0072] An advantageous embodiment of this method includes, after step d / , step e / of fluidly and / or electrically connecting each reactor to the control room and the machinery room, arranging auxiliary circuits, and fluidly and / or electrically connecting them to the nuclear auxiliary building.

[0073] In an advantageous embodiment, the installation in step c / and the arrangement in step d / include passing each hybrid structure in the form of a prefabricated module and each integrated SMR reactor through the same access air lock to the outside from the reactor building where the entire component is discharged in step b / .

[0074] In an advantageous variant, the removal and discharge in step b / are the following consecutive sub-steps, namely, sub-step b1 / of removing the primary line arranged between the steam generator and the reactor vessel; sub-step b2 / of removing and discharging the steam generator; sub-step b3 / of removing and discharging the primary pump; sub-step b4 / of removing and discharging the pressurizer; sub-step b5 / of first removing the primary line from the outlet of the steam generator until it passes through the shell of the reactor building; and include.

[0075] In another advantageous variant, the neutralization of the reactor vessel in step b / is the following consecutive sub-steps, namely, sub-step b6 / of sealing and blocking the hydraulic connection of the reactor vessel; sub-step b7 / of closing the reactor vessel by returning the lid of the reactor vessel and, if necessary, attaching a radiation protection cover; Sub-step of filling the reactor vessel with water or inert gas by means of a connection and level or pressure monitoring device, including.

[0076] Step b6 / preferably consists of placing a solid plug in each hydraulic connection and subsequently seal-welding the plug, the welding being preferably verified by gamma graphics.

[0077] In another advantageous embodiment, step b / includes the step of decontaminating the reactor building after neutralizing the reactor vessel to remove any radioactive contaminants deposited inside the building.

[0078] In another advantageous variant, step c / includes cutting and discharging a part of the shell and / or floor of the infrastructure of the reactor building that initially supports the components of the primary circuit, and if necessary, a part of the raft.

[0079] In another advantageous variant, step c / includes fixing each hybrid structure to the raft of the infrastructure of the reactor building by means of a fixed plate firmly attached or fixed to one and / or the other of the double-skinned metal walls.

[0080] In another advantageous variant, step c / consists of placing the hybrid structure on the raft and fixing it if necessary, and then the following successive sub-steps, namely, Sub-step of cutting and discharging the shell part that separates the reactor vessel well of the LWR reactor forming part of the reactor pool of each hybrid structure, Sub-step of installing a horizontal connection pipe between each hybrid structure and the reactor vessel well, including.

[0081] One variant of this method includes placing at least one shut-off valve, preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, in the pipes after the placement and retention of the integrated SMR reactor in step d / .

[0082] The present invention further has as its object a nuclear power plant obtained by the above-described retrofit method, which comprises a reactor building housing a neutralized LWR reactor vessel and a reactor pool, a nuclear fuel handling system for supplying nuclear fuel assemblies from a fuel building to inside the reactor vessel in the reactor building and vice versa, at least one, preferably three or four, hybrid structures arranged around the neutralized reactor vessel, each hybrid structure comprising an integrated SMR reactor, housing a fuel building, and each integrated SMR reactor being arranged at a position accessible by the fuel handling system, and includes

[0083] The final number of hybrid structures each housing an integrated SMR reactor will depend particularly on the power adaptation required for the power plant to be retrofitted.

[0084] An advantageous embodiment of this power plant further includes a horizontal connection pipe between each hybrid structure and the reactor vessel well, and at least one shut-off valve on the pipe, preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, and the fuel handling system includes at least one device for enabling these transfers of the fuel assemblies through the connection pipe by tilting them one by one from horizontal to vertical.

[0085] In a variant of the advantageous structure, each hybrid structure includes a bottom configured to support the integrated SMR reactor.

[0086] Each hybrid structure is advantageously at least partially filled with water.

[0087] Each hybrid structure is advantageously configured to house a fixed compartment of the SMR reactor and a removable compartment of the SMR reactor when removed from the fixed compartment.

[0088] In an advantageous variant, each hybrid structure is provided with a removable lid that contributes to the function of safely confining the nuclear material.

[0089] Thus, the present invention essentially consists of removing and discharging all components of the primary circuit, except for the LWR reactor vessel in which all materials are emptied and neutralized, and then replacing some of these components with an integrated SMR reactor, a concrete / metal hybrid structure that functions as a reactor vessel well for the SMR reactor and is preferably filled with water, respectively, and fixing the SMR inside the reactor building and preferably rearranging the sub-assemblies that contribute to the third confinement barrier. All this is done with a minimal change to the infrastructure of the reactor building.

[0090] The hybrid structure according to the present invention functions to some extent as a reactor vessel well for the integrated SMR reactor and thus has the following functions, namely, - the function of firmly fixing it to the existing civil engineering infrastructure (raft, shell, and intermediate floor) of the PWR reactor in order to meet the seismic requirements, · the function of being placed in water and providing biological protection, · providing a through connection to the main pool above the existing reactor vessel well of the PWR reactor and a connection to its existing fuel handling system, · placing the entire integrated SMR reactor in a uniform volume of water defined by the internal volume of the hybrid structure and contributing to the function of safely discharging the residual power, - the sealing function provided by its metal double skin that enables this, - contribution to the nuclear material confinement safety function, i.e., by closing with a removable lid at the top of the hybrid structure, the integrated SMR reactor housed and held therein is placed within an enclosure that meets some or all of the requirements related to the nuclear material confinement safety function (the third barrier), - advantageously, because the hybrid structure can be manufactured from prefabricated modules, · The modularity that guarantees insertion module by module in the reactor building, such as in an integrated SMR reactor, enables adaptation of anchor points and connection points to existing civil structures, guarantees that forces are absorbed, and enables compatibility with all PWR reactor configurations. · Assembly by welding, which guarantees great flexibility regarding assembly conditions, and sealing for contribution to a small installation area and a confinement function. · Since there is no shutter itself and no shutter support, optimal insertion into existing infrastructure is possible, the impact of the modification according to the present invention is limited only to what is necessary, and the survival of the site is optimized. A constructability function that enables this. It has.

[0091] In fact, to some extent, the method according to the present invention is at odds with all assumed dismantling methods.

[0092] Substantially, compared with the plan for dismantling a PWR as assumed in Non-Patent Document 3, the present invention Whether it is a building of the nuclear island (reactor building, fuel building or auxiliary building) or a conventional island building (machine room), the building is not demolished. The reactor vessel is not removed from the reactor building. It is not necessary to perform restoration of the site itself. It is distinguished by this fact.

[0093] In other words, the technical reality is that only a few irreplaceable components of the primary circuit of the reactor have reached the end of their legal operating life, but the inventor has overcome the general prejudice in the nuclear power field that the complete dismantling of a nuclear power plant must be carried out to the point of destruction of all buildings and restoration of the site.

[0094] Even if there is a reduction in output, by replacing a PWR reactor and its three or four steam generators with an integrated SMR reactor according to the retrofit method of the present invention, it becomes possible to give a second operating phase to a 900 / 1300 MWe pressurized water reactor (PWR) nuclear power plant.

[0095] The SCOR 200 integrated SMR reactor can usually be designed to output 200 MWe. Also, if a 900 MWe PWR reactor is replaced with three SCOR-type SMR reactors, the output in the second operating phase will be equal to 3×200 / 900 = 67% of its initial output, that is, the power plant will have a 33% reduction in output. For a 1300 MWe PWR reactor, the output reduction will be 38%. Another evaluation of the output in the integrated SMR reactor planned in the NUWARD (trademark) project will also be in the same order of magnitude.

[0096] Ultimately, the retrofit method according to the present invention has a number of advantages as follows.

[0097] Reduction of the initial investment in a nuclear power plant through the reuse of most of the equipment, especially almost all of the conventional island including civil engineering structures and part of the nuclear island. In addition to the partial dismantling according to the present invention, only the renovation of the machinery room is required to adapt to the dimensions of the equipment of the energy conversion cycle in accordance with the reduction in output related to the retrofit.

[0098] There is no need to find a new nuclear power site, which implies a significant reduction in environmental and financial impacts, and the continuity of the surrounding area, economy and social environment of the different existing nuclear power sites where the power plant is converted by this retrofit method is maintained. Furthermore, from a social perspective, the acceptability of the existing nuclear power sites can be considered achieved, and the same can also be applied to the retrofit of these sites.

[0099] Significant shortening of the construction period. The renovation of nuclear power plants using the present invention is carried out according to optimized work procedures, with many operations prepared in advance off-site, which enables at least some of the periods, such as the manufacture of prefabricated modules with a hybrid structure, to be carried out in parallel.

[0100] Significant reduction of waste. By using the maximum amount of the power plant building / equipment in the second operation phase, the amount of waste generated is significantly reduced, including waste with an exceptionally low level of nuclear activity.

[0101] Modularization of the nuclear part of the energy mix. Due to the reduction of output and the modularization of time achieved by the number of reactors to be converted according to the present invention, and especially by their placement within the area in the case of the French installation base, it becomes possible to plan the nuclear part required in the energy mix on a dynamic time scale.

[0102] With the renovation according to the present invention, this form of energy becomes durable, thus improving the reputation of nuclear energy, namely the circular economy of materials and equipment, aging...

[0103] Reduction of the carbon balance of nuclear energy. Most of it is related to the construction of facilities. By increasing the operating time of the facilities (nuclear power plants), the carbon balance per effectively produced MWe is reduced.

[0104] Other advantages and features of the present invention will become more clearly apparent from reading the detailed description of the embodiments of the present invention given as non-limiting examples with reference to the following figures.

Brief Description of the Drawings

[0105]

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Figure 5D

Figure 6

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Figure 14A

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Embodiments for Carrying Out the Invention

[0106] Throughout this application, the terms "vertical," "lower," "upper," "low," "high," "downward," and "upward" are in the context of the reactor building of a power plant and the vertical operating configuration, and are to be understood with reference to an integrated SMR reactor disposed in the reactor building using the retrofit method according to the present invention.

[0107] Figures 1 through 5D are not described below because they have already been described in detail in the preamble.

[0108] For reasons of clarity, the same elements according to the present invention and the prior art are designated by the same reference numerals throughout Figures 1 through 15.

[0109] The various figures do not represent all of the fluid, electrical, and control and command connection or instrumentation systems required for the operation of a nuclear power plant converted by the method according to the present invention. In particular, the fluid lines for steam, and the associated piping, are not mentioned because there are no requirements for the primary integration of these lines in the architecture. In particular, the steam and water fluid supply lines from and to the integrated SMR reactor that need to pass through the hybrid structure are not represented.

[0110] As a preliminary to the description of the method according to the present invention for retrofitting a nuclear power plant, the essential means used and the feasibility of integrating these various means into an existing reactor building are described.

[0111] Figures 6, 6A, and 6B represent a hybrid structure according to the present invention, generally designated by reference numeral 6, which houses therein an integrated SMR reactor generally designated by reference numeral 7, and the whole of which is intended to be installed in place of a subassembly consisting of the primary pumps and steam generators of an existing PWR reactor primary circuit. In these Figures 6, 6A, and 6B, the SCOR type integrated SMR design is selected as a representative.

[0112] The hybrid structure 6 functions to some extent as the reactor vessel well of the integrated SMR reactor 7. Thus, its main functions include housing and supporting such a reactor, the related civil engineering functions (fixing, strength, sealing, constructability), and the advantageous possibility of being able to store the removable compartment 71 of the integrated SMR reactor 7 underwater for the phase of handling fuel or maintaining what is inside the fixed compartment 70 of the SMR.

[0113] The hybrid structure 6 consists of a metal double skin, i.e., two metal walls 60, 61 spaced apart from each other, and the space between these two walls 60, 61 is filled with concrete 62.

[0114] The support floor 63 is arranged substantially horizontally as the bottom inside the inner wall 61 to support the integrated SMR reactor 7.

[0115] The sealed internal volume of the hybrid structure 6 is thus delimited by the inner wall 61 and the bottom 63. This is intended to be filled with water to function as a biological barrier and contributes to the function of discharging the residual power from the SMR according to the configuration of the integrated SMR 6.

[0116] Furthermore, as shown in FIG. 7, the structure 6 can accommodate this internal volume underwater on the side of the fixed compartment 70 of the integrated SMR reactor 7 and is sized such that the removable compartment 71 can be removed from the top of the fixed compartment 70. This enables the safe execution of fuel handling and / or maintenance work in the fixed compartment 70 because the two compartments 70, 71 are in water of uniform volume. The removable compartment 71 is handled by large component handling equipment used for the insertion of the integrated SMR 7.

[0117] As shown in FIG. 8, the hybrid structure 6 is preferably provided with a metal lid 64 removably connected in a sealed manner to one and / or the other of the metal walls 60, 61 of the hybrid structure. When the lid 64 is installed, the structure 6 itself contributes to the safely controlled confinement of nuclear material, and the first consists of a metal sheath that encloses the fuel within the integrated SMR reactor 7, and the second consists of the casing that constitutes the reactor vessel of the integrated SMR reactor 7.

[0118] Furthermore, the hybrid structure 6 has an opening P passing therethrough. As will be described in detail below, this opening P is intended to be connected to a pipe for transferring the fuel assembly from and into the interior of the SMR reactor 7.

[0119] As shown in more detail in FIG. 9, the hybrid structure 6 first includes a metal anchor plate 65 fixed to the raft 41 of the reactor building, thereby making it possible to fix the hybrid structure 6 to the existing infrastructure 4 of the reactor building 1. In the illustrated example, this anchor plate 65 is welded to the inner metal wall 60. In addition to the plate 65 welded to the inner metal wall 60, another anchor plate welded to the outer metal wall 61 can also be clearly assumed. The connection to the raft 41 can be provided using various specific civil engineering techniques, particularly in accordance with the strength requirements obtained from structural studies for loads under seismic conditions.

[0120] Retention bars are welded between the walls 60, 61 inside the double metal skin in the space 62 to maintain the spacing therebetween.

[0121] To reinforce the concrete within the space 62, reinforcing bars 67 are welded to metal studs 66, 68 welded to one and / or the other of the metal walls 60, 61.

[0122] Furthermore, although not shown, in particular, for functioning as a support for the bottom 63 intended to function as a support for the integrated SMR reactor 7 To support the pipes and other auxiliary items necessary for the operation of the integrated SMR reactor 7, connect the hybrid structure 6 to the intermediate floor 42 and the shell 43 of the existing infrastructure 4 of the reactor building 1 so as to mechanically reinforce the whole and achieve an overall strength of the structure at least equivalent to that before implementing the hybrid structure 6. Another plate can be welded to one and / or the other of the metal walls 60, 61.

[0123] FIG. 10 shows a variant of the hybrid structure 6 composed of modules M1, M2, M3, M4 that are prefabricated off-site and then assembled on-site, i.e., inside the reactor building 1. In this variant, the structure 6 can include a mechanical reinforcement 69 arranged at the upper part of the structure. This variant is advantageous because, depending on the type of the reactor building 1 and its existing inlet airlock initially designed for replacing the steam generator 23, the size of each module can be adapted to have the maximum dimension that can be inserted through the inlet airlock. Thereby, the time and cost of the retrofit according to the present invention are further optimized. The hybrid structure has already been introduced internationally for nuclear facilities, and currently, a project has been approved in France. See Non-Patent Document 4.

[0124] Although this is not essential, the retrofit method according to the present invention is advantageously carried out when all the components related to the retrofit are handled without any impact or with a minimal impact on the infrastructure 4 of the reactor building 1, as will be detailed later.

[0125] The inventor analyzed that this implies the ability to remove all the components (primary pump 22, steam generator 23, and pressurizer 24) of the primary circuit of the existing PWR reactor through the airlock provided for this purpose and insert all the most bulky components (hybrid structure 6, integrated SMR reactor 7) of the new primary circuit through the same airlock during the modification phase.

[0126] The possibility of handling the structure 6 before pouring concrete has been demonstrated by the foregoing.

[0127] The inventors have therefore also pre-verified that the integrated SMR reactor 7, i.e., after being fully assembled, can also be handled in a single block via the same handling route, i.e., the inlet airlock of the reactor building.

[0128] Figure 11 relates to an existing steam generator 23 in a PWR reactor. Since the overall dimensions H1*L1 of this type of steam generator 23 are on the order of a height of about 22 m multiplied by a width of 5 m, it is possible to insert it through the existing inlet airlock of the reactor building 10 in order to replace it.

[0129] Figure 12 shows the integrated SMR reactor 7 of the SCOR project. Its overall dimensions H2*L2 are smaller than H1*L1 of the steam generator 23.

[0130] For this example of the integrated SMR reactor 7 project, since the maximum overall dimensions are less than 22 m * 5 m, it is possible to insert it through the inlet airlock in the reactor building 1 by means of the handling system for the steam generator 23 as originally designed. A handling system that positions horizontally and then tilts vertically is also suitable for this example. Similarly, its mass is compatible with the load capacity of the equipment of the handling system.

[0131] As a result, it is achieved that the integrated SMR reactor can be inserted into the reactor building 1 without affecting its infrastructure 4.

[0132] The inventors then considered the optimal arrangement that the hybrid structure 6 should have with the integrated SMR reactor 7 in the reactor building 1.

[0133] In order to optimize the layout and cost of the retrofit method, the inventors used the following integration criteria, namely, limiting the impact on the infrastructure 4 that secures the integrated SMR reactor 7, Maximize the reuse of existing infrastructure elements, the functionality of various barriers, biological protection, etc. Optimize the connection to and functionally integrate an integrated SMR reactor with two existing functional systems, namely, one dedicated to fuel handling and the other dedicated to discharging output to the machinery room. was adopted.

[0134] Based on the above criteria, the layout of the integrated SMR reactor was determined through three-dimensional critical path analysis.

[0135] The optimal arrangement is in terms of height (z), in accordance with the arrangement of the fuel handling / biological protection system, in the (x, y) plane viewed from above, instead of the axisymmetric steam generator 23, the inventors concluded.

[0136] In addition to these two layout parameters, the inventors analyzed that in the operation phase of the integrated SMR reactor, additional space is further required for the removable compartment 71 that must be removed from its fixed compartment 70 for fuel loading / unloading and / or maintenance of internal components.

[0137] In a converted nuclear power plant equipped with three or four integrated SMR reactors, it is preferable to consider the removable compartment 71 and the reactor layout equally.

[0138] By analyzing the spatial configuration of the current primary circuit of the PWR, the inventor has found this optimal layout. In fact, in each primary loop 21, the primary pump 22 is spatially back-to-back with the steam generator 23 it is associated with. Therefore, if the integrated SMR reactor 7 is installed in place of the steam generator 23, it is possible to secure the space occupied by the primary pump 22 with a removable compartment 71. This optimal configuration is schematically shown in FIG. 13. Finally, it becomes possible to allocate a dedicated upper removable location for each SMR, and it becomes possible to consider an installation work configuration where all SMRs are required to be opened simultaneously.

[0139] Next, referring to FIGS. 14A to 14E, various steps of the method according to the present invention for retrofitting an existing PWR nuclear power plant will be described in consideration of the above analysis.

[0140] Step a / : The PWR is shut down.

[0141] This first step aims to enable the configuration of the retrofit power plant site.

[0142] All the fuel assemblies present in the reactor vessel 20 are discharged outside the reactor building 1. The primary circuit 2 is then completely drained.

[0143] The safety analysis before opening the site can show whether the fuel assemblies can remain in the pool of the fuel building during the duration of the site. In this case, the retrofit site (steps b / and c / ) can be opened without waiting for the fuel assemblies to reach a residual power that complies with the regulations for the transport of nuclear materials, thus saving time for the completion of the work plan.

[0144] Step b / : Partial electromechanical disassembly of the PWR is performed. The components of the primary circuit 2 are thus, preferably, in the following successive sub-steps, namely, b1 / Disassembly of the primary line 21 arranged between the steam generator 23 and the reactor vessel 20, b2 / Dismantling and discharging of the steam generator 23, b3 / Dismantling and discharging of the primary pump 22, b4 / Dismantling and discharging of the pressurizer 24, b5 / Dismantling of the primary line 21 from the outlet of the steam generator 23 until it passes through the shell of the reactor building at first, are dismantled and discharged outside the reactor building 1.

[0145] Only the reactor vessel 20 remains in the reactor building 1 as it is (FIG. 14A). In fact, leaving the reactor vessel 20 as it is does not prevent achieving the retrofit installation configuration. Furthermore, the inventor has found that leaving the reactor vessel as it is during the phase of operating a nuclear power plant retrofitted with an integrated SMR reactor results in a period of time during which the activation substances, particularly Co 60 decreases.

[0146] On the other hand, all substances are removed from the inside of the reactor vessel 20, and then the reactor vessel 20 is neutralized.

[0147] For this purpose, the following sub-steps, namely, b6 / If possible, place solid plugs at each hydraulic connection, and then seal and weld the plugs to block and seal the hydraulic connections of the reactor vessel. The welded parts are preferably verified by gamma graphics, b7 / Reattach the lid of the reactor vessel, pre-close all passages for control rods, and close the reactor vessel by attaching a radiation protection cover if necessary, b8 / Fill the reactor vessel with water or an inert gas by means of a connection and pressure control and / or liquid level device. The filling and liquid level control device will be arranged inside the reactor building. In particular, it can be connected to the reactor vessel by reusing one or more of the passages through the lid to provide a fluid connection, are carried out.

[0148] If necessary, the lid of the reactor vessel 20 can be changed, in particular to perfect its sealing and / or to enable optimization of the neutralization of the reactor vessel.

[0149] After the neutralization of the reactor vessel 20, if necessary, the interior of the reactor building 1 is decontaminated to remove any radioactive contaminants that may have accumulated.

[0150] Taking radiation protection into account, steps a / and b / are carried out either by human intervention or remotely.

[0151] Step c / : The hybrid structure 6 is installed.

[0152] Prior to this step c / , in particular all connections of the infrastructure 4 of the reactor building 1 to the hybrid structure 6, the dimensions of the hybrid structure 6, usually the thickness of the plates for the walls 60, 61, the density and dimensions between the walls 60, 61 and the studs and connecting rods, the method of fixing to the raft 41, and the connection to the floor 43 and the shell 42 connected to the hybrid structure 6 can be defined by performing a seismic strength investigation of the overall nuclear island configuration.

[0153] This step c / includes cutting and discharging a part of the shell 42 and / or the floor 43 of the infrastructure 4 of the reactor building 1 and, if necessary, the raft 41.

[0154] This enables the curing of the hybrid structures 6 and the preparation for providing all the devices for fixing them to the infrastructure 4.

[0155] Furthermore, step c / consists of creating an opening in the shell 42 leading to the existing lower pool of the reactor for connection to the fuel handling system. Core drilling technology is advantageously used for this operation.

[0156] Figure 14B shows the space E that needs to be opened for the installation of the hybrid structure 6 A circular opening O connected to the pool above the reactor vessel 20 is shown.

[0157] All cutting operations can be carried out using a concrete cutting device that is already widely used in nuclear practice. Operations for preparing the existing infrastructure 4 can also be carried out.

[0158] When these operations of cutting the infrastructure and discharging the cut parts are carried out, a hybrid structure consisting of prefabricated modules enters through the inlet airlock of the reactor building 1. The modules can usually be introduced in the form of horizontal sections with a unit height of 5 meters.

[0159] Subsequently, the hybrid structure 6 is properly arranged. This arrangement involves fixing it to the infrastructure 4 of the reactor building 1. In particular, each hybrid structure 6 is fixed to the raft 41 by a fixing plate 65. The prefabricated modules are welded to each other, and anchor connections are made to the shell 42 and the floor 43. Furthermore, sealing is performed for the compartment under the reactor vessel 20.

[0160] When each fixing device of the hybrid structure 6 is arranged and fixed in place, a metal horizontal connection pipe 80 is installed between each hybrid structure and the reactor vessel well 20, preferably by seal welding to two metal walls 60, 61 of a double casing. On the side of the pool above the reactor vessel 20, to ensure the sealing of the pipe 80, it is welded to the pool liner. This pipe 80 is a transfer pipe through which a fuel assembly can be handled by a handling system.

[0161] Step d / : The integrated SMR reactor 7 is arranged and held in each hybrid structure 6 installed in step c / .

[0162] As described above, the integrated SMR reactor 7 is arranged at a position accessible by an existing fuel handling system.

[0163] Each integrated SMR reactor 7 initially manufactured off-site is introduced into the reactor building by an existing handling system and placed directly on the bottom 63 of the hybrid structure 6 provided for this purpose.

[0164] Finally, shut-off valves 81, 82 are installed at the ends of each pipe 80 (Figs. 14D, 14E).

[0165] Step e / : Next, the fluid and / or electrical connections to the control room and machine room of each integrated SMR reactor 7 continue.

[0166] Auxiliary circuits are installed and fluid and / or electrical connections to the nuclear auxiliary building are made.

[0167] FIG. 15 shows the internal architecture of the reactor building 1 of an initial PWR power plant after being converted using the retrofit method of the present invention, with three hybrid structures 6 each housing and supporting an integrated SMR reactor 7.

[0168] The present invention is not limited to the examples described herein, and in particular, the features of the illustrated examples can be combined with each other in variations not shown.

[0169] Other variations and embodiments can be envisioned without departing from the scope of the present invention.

[0170] In the illustrated example, the hybrid structure is sized to optimize the integration of the integrated SMR reactor 7 and its removable compartment 71 during operation, but it is also possible to envision solutions for reducing the dimensions of the hybrid structure, i.e., the mutual layout for all removable compartments 71 after their respective fixed compartments 70 have been removed.

[0171] In the context of the present invention, it can be assumed that the removable compartment of the integrated SMR reactor is handled at the bottom of the hybrid structure or, at least, under the same water level beside the fixed compartment of the SMR.

[0172] In the illustrated example, the means for transferring from the integrated SMR reactor 7 to the pool above the reactor vessel 20 is limited to a single pipe 80 so that valves 81, 82 can isolate different volumes of water (the internal volume of the hybrid structure 6, the pool above the reactor vessel 20). In this option, since the fuel assembly has to be transferred horizontally and thus has to be introduced horizontally into the pipe 80 once it has been vertically extracted from inside the integrated SMR reactor 7, it is necessary to provide a vertical / horizontal tilting device. This horizontal position can be maintained until the assembly exits the reactor building 1 as the assembly is transferred to the fuel building at this position.

[0173] As a variant, the transfer pipe 80 can also be replaced by a weir or a free surface water channel equipped to isolate its volume of water. The weir has the function of a valve in the sense that it enables hydraulic isolation between the two compartments it separates. This type of device makes it possible to omit the horizontal / vertical tilting device.

[0174] The illustrated example of the retrofit method relates to a PWR reactor. This type of method can equally function as a basis for a method of retrofitting a BWR reactor, subject to adaptations related to the specific configuration of that type of reactor compared to a PWR, and these changes will be apparent to those skilled in the art of reactor technology.

Explanation of reference numerals

[0175] 1 Reactor building 10 Inner wall 12 Outer wall 13 Annular space 20 Reactor vessel 22 Primary pump 23 Steam generator 4 Infrastructure 41 Raft 42 Shell 43 Floor 6 Hybrid Structure 60 Internal Metal Wall 61 External Metal Wall 62 Space 63 Bottom 64 Lid 65 Anchor Plate 66 Metal Stud 67 Reinforcing Bar 69 Reinforcement 7 Integrated SMR Reactor 70 Fixed Compartment 71 Removable Compartment 80 Pipe 81, 82 Shut-off Valve

Claims

1. A method for retrofitting a nuclear power plant initially including at least one light water reactor (LWR), in particular a pressurized water reactor (PWR) or a boiling water reactor (BWR), comprising a reactor vessel (20), a primary circuit (2), a reactor building (1) housing a reactor pool, a fuel building, a nuclear fuel handling system for supplying nuclear fuel assemblies from the fuel building to the inside of the reactor vessel and vice versa, a machinery room (5), a control room, and an auxiliary nuclear building, wherein for each reactor, a / a step of shutting down the reactor, including discharging all the fuel assemblies present in the reactor vessel (20) outside the reactor building and completely draining the primary circuit (2); b / a step of partially electro-mechanically disassembling the reactor, including removing and discharging the components (21, 22, 23, 24) of the primary circuit outside the reactor building, and leaving the reactor vessel (20) of the reactor as it is in the reactor building, the step of neutralizing the reactor vessel following the removal of all materials from the inside of the reactor vessel; c / a step of installing at least one removable closed hybrid structure (6) consisting of a metal double skin (60, 61) and concrete injected into the space (62) between two metal walls constituting the double skin, replacing some of the components of the primary circuit discharged in step a / ; d / a step of placing and holding at least one reactor (7) called an integrated small modular reactor (SMR) inside each hybrid structure installed in step c / , the integrated SMR reactor being placed at a position accessible by the fuel handling system; A method comprising the above steps.

2. The retrofit method according to claim 1, further comprising step e / of fluidly and / or electrically connecting each reactor to the control room and the machinery room after step d / , arranging an auxiliary circuit, and fluidly and / or electrically connecting to the auxiliary nuclear building.

3. The installation in step c / and the placement in step d / involve passing each hybrid structure and each integrated SMR reactor in the form of a prefabricated module through the same access air lock to the outside from the reactor building where the entire component is discharged in step b / , the retrofit method according to claim 1 or 2.

4. The disassembly and discharge in step b / are the following consecutive sub-steps, namely, b1 / a sub-step of disassembling the primary line (21) arranged between the steam generator (23) and the reactor vessel (20); b2 / a sub-step of disassembling and discharging the steam generator (23); b3 / a sub-step of disassembling and discharging the primary pump (22); b4 / a sub-step of disassembling and discharging the pressurizer (24); b5 / a sub-step of first disassembling the primary line (21) from the outlet of the steam generator until it passes through the shell of the reactor building; The retrofit method according to any one of claims 1 to 3, comprising.

5. The neutralization of the reactor vessel in step b / is the following consecutive sub-steps, namely, b6 / a sub-step of sealing and blocking the hydraulic connection part of the reactor vessel; b7 / a sub-step of closing the reactor vessel by returning the lid of the reactor vessel and, if necessary, attaching a radiation protection cover; b8 / a step of filling the reactor vessel with water or an inert gas by a connection and level or pressure monitoring device; The retrofit method according to any one of claims 1 to 4, comprising.

6. Step b6 / consists of placing a solid plug in each hydraulic connection part and subsequently sealing and welding the plug, and the welding is preferably verified by gamma graphics, the retrofit method according to claim 5.

7. Step b / includes, after the neutralization of the reactor vessel, decontaminating the reactor building to remove any radioactive contaminants deposited inside the building, the retrofit method according to any one of claims 1 to 6.

8. Step c / includes cutting and discharging a part of the shell (42) and / or the floor (43) of the infrastructure (4) of the reactor building that initially supports the components of the primary circuit, and, if necessary, the raft (41), the retrofit method according to any one of claims 1 to 7.

9. Step c / preferably includes fixing each hybrid structure to the raft of the infrastructure of the reactor building by means of a fixing plate (65) firmly attached or fixed to one and / or the other of the metal walls of the double outer skin, according to any one of claims 1 to 8.

10. Step c / includes arranging the hybrid structure on the raft and fixing it if necessary, and then the following successive sub-steps, namely, a sub-step of cutting and discharging a shell portion that separates the reactor vessel well of the LWR reactor forming part of the reactor pool of each hybrid structure; a sub-step of installing a horizontal connection pipe (80) between each hybrid structure and the reactor vessel well; and is included in the retrofit method according to any one of claims 1 to 9.

11. After the placement and retention of the integrated SMR reactor in step d / , arranging at least one shut-off valve (81, 82), preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, in the pipe, according to the retrofit method according to claim 10.

12. A reactor building (1) housing a neutralized LWR reactor vessel (20) and a reactor pool; A nuclear fuel handling system for supplying nuclear fuel assemblies from the fuel building to the reactor vessel inside the reactor building and vice versa; At least one, preferably three or four hybrid structures (6) arranged around the neutralized reactor vessel, each hybrid structure comprising an integrated SMR reactor (7) and housing a fuel building, and each integrated SMR reactor being arranged at a position accessible by the fuel handling system; A nuclear power plant obtained by the retrofit method according to any one of claims 1 to 11.

13. Further comprising a horizontal connection pipe (80) between each hybrid structure and the reactor vessel well, and at least one shut-off valve (81, 82) on the pipe, preferably two shut-off valves, one on the hybrid structure side and the other on the reactor vessel well side, and the fuel handling system includes at least one device for enabling the transfer of fuel assemblies through the connection pipe by tilting the fuel assemblies one by one from horizontal to vertical.

14. The nuclear power plant according to claim 12 or 13, wherein each hybrid structure includes a bottom (63) configured to support an integrated SMR reactor (7).

15. The nuclear power plant according to any one of claims 12 to 14, wherein each hybrid structure is at least partially filled with water.

16. The nuclear power plant according to any one of claims 12 to 15, wherein each hybrid structure is configured to accommodate a fixed compartment (70) of the SMR reactor and a removable compartment (71) of the SMR reactor when removed from the fixed compartment.

17. The nuclear power plant according to any one of claims 12 to 16, wherein each hybrid structure is provided with a removable lid (64) that contributes to the nuclear material confinement control safety function.

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